Your DPS sounds fine, depending on how your RNG is. Without logs we can't evaluate your combustions, so just looking at your armory:

1) Switch your int/haste gems to int/crit.
2) Your haste is generally too high, unless you're shooting for a higher haste plateau, with 2 Piece T13 you should be aiming for 1505 haste
3) Even if you are shooting for a different haste plateau, you're a fair bit over hit cap. I'd recommend wowreforge.com or a reforge addon like reforgelite to be a bit more efficient with your stats.

38k dps is fine but since you said your best round is around 40% i think its fine

remember fire mages get 12% damage bonus when target is below <35% so you should go up to 40k dps ish when the boss dies but aside from that fire is just rng dps -_- which i really hate unlike arcane with pure mana management skill and cd timing but yea just hope for 75% uptime on ignite and heaps of pyroblast you will hit around 45k dps

Your DPS sounds fine, depending on how your RNG is. Without logs we can't evaluate your combustions, so just looking at your armory:

1) Switch your int/haste gems to int/crit.
2) Your haste is generally too high, unless you're shooting for a higher haste plateau, with 2 Piece T13 you should be aiming for 1505 haste
3) Even if you are shooting for a different haste plateau, you're a fair bit over hit cap. I'd recommend wowreforge.com or a reforge addon like reforgelite to be a bit more efficient with your stats.

Thank You man.

First off all I have a bit over hit cap because I looted 397 chest last night without gem/regorge yet.

Im using reforgelite, but I try to reach 2005 haste rating.

If you only aim for 1505, + 500 haste 2xT13, you wait to cast Combustion until got 10 stack of Stolen Time or cast as soon as possible?

I had the same problem , i was in a guild doing about 32-36k dps on 80% of the tries , dipping to 30k or going to 41k depending on rng and i could not undestand why my dps was sooooo low, was really frustrating.
i then went and did it on 10 man with about 10% less crit due to buffs and did 38.9k with no worries. really seemed to do so much more dmg with the same gear in 10 man compared to 25.

First off all I have a bit over hit cap because I looted 397 chest last night without gem/regorge yet.

Im using reforgelite, but I try to reach 2005 haste rating.

If you only aim for 1505, + 500 haste 2xT13, you wait to cast Combustion until got 10 stack of Stolen Time or cast as soon as possible?

Depends on the fight, and how your guild uses heroism. Personally, on Ultraxion (Normal, to be clear), my guild uses heroism to start the fight. That means that so long as I overlap my 1st combustion with an insignia proc I'll get a large amount of ticks, independent of how many stacks of stolen time I have. On a fight like Ultraxion, even without heroism, it's likely a good idea to just combust with your insignia proc + a decent ignite.

First off all I have a bit over hit cap because I looted 397 chest last night without gem/regorge yet.

Im using reforgelite, but I try to reach 2005 haste rating.

If you only aim for 1505, + 500 haste 2xT13, you wait to cast Combustion until got 10 stack of Stolen Time or cast as soon as possible?

As someone said, check the spreadsheet. For the first combustion your best bet is to use it with a good ignite + insignia proc. That being said, if your guild is using heroism early or you've got some weird spread of buffs, then check the spreadsheet and see if you're going to be missing/gaining any additional ticks without waiting for a certain stack. The combustion plateau's are spaced enough that I don't think there will ever be a time, with an insignia proc, where having any number of stacks will get you another tick.